Data Hacking

196 Part III — Data Hacking
NMEA Sentences
The concept behind NMEA is to send data one line at a time. Each line of data (called a sentence)
is completely standalone — self-contained and independent from other sentences. There are two
kinds of sentences:
Standard sentences for each device category
Proprietary sentences for use by the individual devices and manufacturers
All of the standard sentences have a two-letter prefix defining the device that uses that sentence
type. For GPS receivers, the prefix is always GP — most sentences that a GPS receiver
understands are prefixed by these two letters. They are followed by a three-letter sequence that
defines what the sentence contains.
As I’ve already mentioned, the NMEA standard permits hardware manufacturers to define and
use their own proprietary sentences for any purpose they see fit. The only prerequisite is that all
proprietary sentences must begin with the letter P, and are followed with three letters that
identify the manufacturer controlling that sentence. For example, Garmin sentences start with
“PGRM,” while Magellan sentences begin with “PMGN.”
Proprietary sentences vary from receiver to receiver and over time and are not covered here.
Proprietary sentences contain more, and more specific, information than NMEA does, which is
considered the lowest common denominator.
NMEA Sentence Structure
Each NMEA sentence begins with a dollar sign ($) and ends with a carriage return/line feed
sequence. Most sentences can be no longer than 80 characters. All the data is contained inside
this single line, and data items are separated by commas. The data itself is in the form of
ASCII text, and the data can, especially if it’s complex, be spread over many separate NMEA
sentences, but it is normally fully contained in just one sentence.
The precision of different sentences varies depending on the data. For example, time information
might be given in seconds correct to one decimal place, while positional information might be
given to three or even four digits after the decimal place. NMEA uses commas to separate individual
fields of data. If the field doesn’t contain data, it’s left blank, but a comma is still required.
To improve accuracy, there is a provision to include a checksum at the end of each sentence,
which may or may not be checked by the unit that reads the data. The checksum field consists
of an * and two hex digits representing the exclusive OR of all characters between, but not
including, the $ and *. The checksum is used to validate the contents of the sentence.
A checksum is required on some sentences, and optional on others.